Laboratory Study of Low-Frequency Waves Induced by Random Gravity Waves on Sloping Beaches
Publication: Journal of Waterway, Port, Coastal, and Ocean Engineering
Volume 136, Issue 3
Abstract
Low-frequency waves induced by the shoaling of random gravity waves (the primary waves) on sloping beaches were studied experimentally. Incident primary waves with a Pierson-Moskowitz (PM) spectrum were mechanically generated at a water depth of 0.7 m and propagated toward beaches with three different slopes (1/20, 1/30, and 1/40) in separate experiments. Four wave conditions with significant wave heights ranging from 0.078 to 0.125 m were used. The time series of surface elevations were simultaneously recorded at various water depths along the beach. Low-frequency waves were obtained from the data with a low-pass filter. Our results show that the spectra of low-frequency waves on beaches are dramatically changed by the beach slope and the wave height of incident primary waves. The cross-correlation of low-frequency waves with the surface-elevation envelope of the primary waves shows that the incident bound long waves are dominant. Free low-frequency waves propagate in the offshore direction with very low amplitudes due to high energy dissipation on the beach. The energy ratio between the low-frequency and the primary waves in the shoaling region outside the surf zone strongly correlates to the local surface skewness. For a given water depth, the low-frequency wave energy is proportional to , where is the primary wave energy, in agreement with the field observations.
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Acknowledgments
The writers are indebted to Dr. J. H. Duncan and Professor J. A. Battjes for their valuable comments and review of this paper and to Dr. T. H. C. Herbers for providing the numerical code. This work is supported by National Science Foundation, Division of Ocean Sciences (Grant No. NSFOCE0221335), National Natural Science Foundation of China (Grant No. NNSFC49606071), State Oceanic Administration of China, Ocean Science Foundation for Young Scientists (Grant No. UNSPECIFIED95409) and the Open Fund of Physical Oceanography Laboratory at Ocean University of China. The experiment was carried out in the Physical Oceanography Laboratory at Ocean University of China.
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Information
Published In
Journal of Waterway, Port, Coastal, and Ocean Engineering
Volume 136 • Issue 3 • May 2010
Pages: 127 - 134
Copyright
© 2010 ASCE.
History
Received: Jan 16, 2009
Accepted: Sep 1, 2009
Published online: Sep 23, 2009
Published in print: May 2010
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